WO 03/107295 A1 shows a monitoring system for status monitoring of peripheral apparatus, for example elevator components. For that purpose the bus system has a bus, a central control unit, which is connected with the bus, and several peripheral apparatus. Each of these apparatus lies at a bus junction and communicates with the control unit by means of the bus. At every point in time the peripheral apparatus adopt a specific status. The control unit periodically interrogates the states of each peripheral apparatus by way of the bus.
The bus is supplied with energy by the control unit and supplies electromagnetic induction loops which are part of a bus junction. The individual peripheral apparatus are coupled to the induction loops of the bus junctions by way of a local antenna and draw electromagnetic energy via the associated induction loop. By way of the induction loop the peripheral apparatus also informs the control unit its identification code and its instantaneous status when each interrogation takes place. Thanks to this identification code the control unit can allocate the read status to a specific peripheral apparatus.
The advantage of such a monitoring system is the simple connection between the bus and peripheral apparatus by means of the induction loops. A complicated and expensive cabling of the peripheral apparatus is redundant.
However, the periodic interrogation of the status of the peripheral apparatus by way of the bus has a disadvantageous effect. Since the control unit actively interrogates each peripheral apparatus the bus communicates two signals for each interrogation and peripheral apparatus. In the case of relatively short interrogation cycles, particularly with safety-relevant peripheral apparatus, and a relatively high number of such apparatus a multiplicity of signals is exchanged between the control unit and peripheral apparatus. This means that the control unit has high computing capacities in order to process all signals. In addition, the bus is strongly loaded and provides high signal transmission capacities in order to communicate all status interrogations. Accordingly, the control unit and the bus are expensive.